U.S. patent application number 10/353331 was filed with the patent office on 2004-03-25 for process for the production of low benzene gasoline.
This patent application is currently assigned to CATALYTIC DISTILLATION TECHNOLOGIES. Invention is credited to Groten, Willibrord A., Rock, Kerry L..
Application Number | 20040055933 10/353331 |
Document ID | / |
Family ID | 32849513 |
Filed Date | 2004-03-25 |
United States Patent
Application |
20040055933 |
Kind Code |
A1 |
Groten, Willibrord A. ; et
al. |
March 25, 2004 |
Process for the production of low benzene gasoline
Abstract
A process for the production of low benzene content gasoline is
disclosed wherein a full boiling range naphtha is fractionated to
produce a light naphtha, a medium naphtha and a heavy naphtha. The
benzene is contained in the medium naphtha and this stream is
subjected to hydrogenation to convert the benzene to cyclohexane
which may be isomerized to improve the octane. The valuable olefins
are removed in the light naphtha and the valuable heavier aromatics
(toluene and xylenes) are removed in the heavy naphtha. In a
preferred embodiment all of the reactions are carried out in
distillation column reactors.
Inventors: |
Groten, Willibrord A.;
(Houston, TX) ; Rock, Kerry L.; (Houston,
TX) |
Correspondence
Address: |
KENNETH H. JOHNSON
P.O. BOX 630708
HOUSTON
TX
77263
US
|
Assignee: |
CATALYTIC DISTILLATION
TECHNOLOGIES
|
Family ID: |
32849513 |
Appl. No.: |
10/353331 |
Filed: |
January 29, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60411810 |
Sep 18, 2002 |
|
|
|
Current U.S.
Class: |
208/57 ; 208/49;
208/85; 208/92 |
Current CPC
Class: |
C10G 2400/02 20130101;
C10G 49/002 20130101 |
Class at
Publication: |
208/057 ;
208/092; 208/049; 208/085 |
International
Class: |
C10G 051/02; C10G
055/02; C10G 057/00; C10B 057/02; C10G 065/02; C10G 067/02 |
Claims
The invention claimed is:
1. A process for producing low benzene content gasoline comprising
the steps of: (a) feeding a full boiling range naphtha containing
benzene to a distillation column wherein a light naphtha fraction
is taken as overheads, a medium naphtha fraction containing said
benzene is taken as a side draw and a heavy naphtha fraction is
taken as a bottoms; (b) feeding said medium naphtha fraction
containing said benzene to a hydrogenation reactor containing a
hydrogenation catalyst wherein a portion of said benzene is
hydrogenated to cyclohexane; and (c) combining said overheads, said
bottoms and the effluent from said hydrogenation reactor to produce
a gasoline lower in benzene content than said full boiling range
naphtha feed.
2. The process according to claim 1 wherein said full boiling range
naphtha further contains olefins, diolefins, mercaptans, thiophenes
and other organic sulfur compounds and said thiophenes are
contained in said medium naphtha fraction and are converted to
hydrogen sulfide in said hydrogenation reactor.
3. The process according to claim 2 further comprising the steps
of: (d) feeding said overheads to a thioetherification reactor
containing a thioetherification catalyst wherein a portion of said
diolefins are reacted with a portion of said mercaptans to produce
sulfides; (e) separating said sulfides from said overheads; (f)
feeding said bottoms and hydrogen to a hydrodesulfurization reactor
containing a hydrodesulfurization catalyst wherein a portion of
said other organic sulfur compounds are reacted with hydrogen to
produce hydrogen sulfide; and (g) separating said hydrogen sulfide
from said bottoms.
4. The process according to claim 3 wherein the effluent from said
hydrogenation reactor is fed to an isomerization reactor containing
an isomerization catalyst wherein a portion of said cyclohexane is
isomerized to methyl cyclo pentane.
5. The process according to claim 4 wherein said distillation
column contains said thioetherification catalyst and said sulfides
are removed along with said bottoms.
6. The process according to claim 4 wherein said hydrogenation
catalyst is contained within a second distillation column and any
material boiling at a temperature below that of benzene or
cyclohexane is removed as a second overheads and any material
boiling at or above the boiling point of benzene and cyclohexane is
removed as a second bottoms.
7. The process according to claim 4 wherein said
hydrodesulfurization catalyst is contained within a third
distillation column and the hydrodesulfurization reaction is
carried out simultaneously with distillation.
8. The process according to claim 4 wherein said isomerization
catalyst is contained within a fourth distillation column and the
isomerization reaction is carried out simultaneously with
distillation.
9. A process for the production of low sulfur, low benzene content
gasoline comprising the steps of: (a) feeding hydrogen and a full
boiling range naphtha containing olefins, benzene, diolefins,
mercaptans, thiophenes and other organic sulfur compounds to a
first distillation column reactor containing a thioetherification
catalyst; (b) concurrently in said first distillation column
reactor; (i) contacting said diolefins with said mercaptans in the
presence of said thioetherification catalyst to react a portion of
said diolefins with a portion of said mercaptans to produce
sulfides, and (ii) fractionating said full boiling range naphtha to
produce a light naphtha, a medium naphtha containing said benzene
and said thiophene and a heavy naphtha; (c) removing said light
naphtha from said first distillation column reactor as a first
overheads; (d) removing said medium naphtha from said first
distillation column reactor as a side draw; (e) removing said heavy
naphtha from said first distillation column reactor as a first
bottoms; (f) feeding hydrogen and said medium naphtha to a second
distillation column reactor containing a hydrogenation catalyst;
(g) concurrently in said second distillation column reactor; (i)
contacting the benzene and thiophene contained within said medium
naphtha with hydrogen in the presence of said hydrogenation
catalyst to hydrogenate a portion of said benzene to cyclohexane
and to react a portion of said thiophene with said hydrogen to
produce hydrogen sulfide, and (ii) fractionating said medium
naphtha to separate said hydrogen sulfide and any material boiling
at a temperature below that of benzene or cyclohexane from any
material boiling at or above the boiling point of benzene and
cyclohexane; (h) removing said hydrogen sulfide and the material
boiling a temperature below that of benzene or cyclohexane as a
second overheads; and (i) removing any material boiling at or above
the boiling point of benzene and cyclohexane as a second
bottoms.
10. The process according to claim 9 further comprising feeding
said second bottoms to a third distillation column reactor
containing an isomerization catalyst and concurrently in said third
distillation column reactor: (a) isomerizing a portion of said
cyclohexane to methyl cyclo pentane to form a reaction mixture; and
(b) separating the isomerization product from the reaction mixture
by fractional distillation.
11. The process according to claim 9 further comprising feeding
hydrogen and said first bottoms to a fourth distillation column
reactor containing a hydrodesulfurization catalyst and concurrently
in said fourth distillation column reactor: (a) reacting a portion
of said heavier organic sulfur compounds with hydrogen to form
hydrogen sulfide; and (b) separating hydrogen sulfide from the
heavy naphtha by fractional distillation.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a process for the
production of low benzene content gasoline. More particularly the
invention relates to a process wherein a full boiling range naphtha
is fractionated to separate out a light naphtha fraction, a medium
naphtha fraction containing the benzene and a heavy naphtha. More
particularly the invention relates to a process wherein the medium
naphtha is hydrogenated to convert the benzene to cyclohexane.
[0003] 2. Related Information
[0004] Petroleum distillate streams contain a variety of organic
chemical components. Generally the streams are defined by their
boiling range which determines the composition. The processing of
the streams also affects the composition. For instance, products
from either catalytic cracking or thermal cracking processes
contain high concentrations of olefinic materials as well as
saturated (alkanes) materials, aromatic compounds and
polyunsaturated materials (diolefins). Additionally, these
components may be any of the various isomers of the compounds.
[0005] The composition of untreated naphtha as it comes from the
crude still, or straight run naphtha, is primarily influenced by
the crude source. Naphthas from paraffinic crude sources have more
saturated straight chain or cyclic compounds. As a general rule
most of the "sweet" (low sulfur) crudes and naphthas are
paraffinic. The naphthenic crudes contain more unsaturates and
cyclic and polycylic compounds. The higher sulfur content crudes
tend to be naphthenic. Treatment of the different straight run
naphthas may be slightly different depending upon their composition
due to crude source.
[0006] Reformed naphtha or reformate generally requires no further
treatment except perhaps distillation or solvent extraction for
valuable aromatic product removal. Reformed naphthas have
essentially no sulfur contaminants due to the severity of their
pretreatment for the process and the process itself.
[0007] Cracked naphtha as it comes from the catalytic cracker has a
relatively high octane number as a result of the olefinic and
aromatic compounds contained therein. In some cases this fraction
may contribute as much as half of the gasoline in the refinery pool
together with a significant portion of the octane.
[0008] Catalytically cracked naphtha gasoline boiling range
material currently forms a significant part (=1/3) of the gasoline
product pool in the United States and it provides the largest
portion of the sulfur. The sulfur impurities may require removal,
usually by hydrotreating, in order to comply with product
specifications or to ensure compliance with environmental
regulations. Some users require the sulfur of the final product to
be below 50 wppm. In addition the EPA requires that the benzene
content of the gasoline be low, i.e., 1 vol. %.
[0009] The most common method of removal of the sulfur compounds is
by hydrodesulfurization (HDS) in which the petroleum distillate is
passed over a solid particulate catalyst comprising a hydrogenation
metal supported on an alumina base. Additionally copious quantities
of hydrogen are included in the feed. The following equations
illustrate the reactions in a typical HDS unit:
RSH+H.sub.2---.notlessthan.RH+H.sub.2S (1)
RCl+H.sub.2---.notlessthan.RH+HCl (2)
2RN+4H.sub.2---.notlessthan.2RH+2NH.sub.3 (3)
ROOH+2H.sub.2---.notlessthan.RH+2H.sub.2O (4)
[0010] Typical operating conditions for the HDS reactions are:
1 Temperature, .degree. F. 600-780 Pressure, psig 600-3000 H.sub.2
recycle rate, SCF/bbl 1500-3000 Fresh H.sub.2 makeup, SCF/bbl
700-1000
[0011] After the hydrotreating is complete, the product may be
fractionated or simply flashed to release the hydrogen sulfide and
collect the now desulfurized naphtha. The loss of olefins by
incidental hydrogenation is detrimental by the reduction of the
octane rating of the naphtha and the reduction in the pool of
olefins for other uses.
[0012] Generally refiners tend to prevent benzene from entering the
gasoline blending stock. For example as mentioned above the cracked
naphthas may be subjected to aromatic removal by solvent
extraction. This, however, removes all aromatic material not just
the benzene. One method of preventing the introduction of benzene
into the gasoline pool is to remove the benzene precursor
(isohexane) from the charge to the catalytic reforming units. This
does not solve the problem of streams which contain benzene as well
as heavier aromatic compounds such as toluene and xylenes. The
heavier aromatics contribute greatly to the octane pool and to date
have not been found to be detrimental to the environment.
[0013] U.S. Pat. No. 5,7734,670 discloses a process for the
hydrogenation of aromatics in a petroleum stream. However, like
solvent extraction, the process is not selective to only the
benzene. U.S. Pat. No. 5,856,602, discloses the hydrogenation of
aromatics in a hydrocarbon stream utilizing a distillation column
reactor wherein the placement of the catalyst bed and operation of
the distillation column controls which aromatic is retained in the
catalyst bed for hydrogenation. U.S. Pat. No. 6,187,980 B1
discloses a process for the hydrogenation of benzene to cyclohexane
in a distillation column reactor wherein essentially pure benzene
is used as the feed to the reactor.
[0014] In addition to supplying high octane blending components the
cracked naphthas are often used as sources of olefins in other
processes such as etherification. The conditions of hydrotreating
of the naphtha fraction to remove sulfur will also saturate some of
the olefinic compounds in the fraction reducing the octane and
causing a loss of source olefins.
[0015] Various proposals have been made for removing sulfur while
retaining the more desirable olefins. Since the olefins in the
cracked naphtha are mainly in the low boiling fraction of these
naphthas and the sulfur containing impurities tend to be
concentrated in the high boiling fraction the most common solution
has been prefractionation prior to hydrotreating. The
prefractionation produces a light boiling range naphtha which boils
in the range of C.sub.5 to about 250.degree. F. and a heavy boiling
range naphtha which boils in the range of from about
250-450.degree. F.
[0016] The predominant light or lower boiling sulfur compounds are
mercaptans while the heavier or higher boiling compounds are
thiophenes and other heterocyclic compounds. The separation by
fractionation alone will not remove the mercaptans. In the past the
mercaptans have been removed by oxidative processes involving
caustic washing. A combination oxidative removal of the mercaptans
followed by fractionation and hydrotreating of the heavier fraction
is disclosed in U.S. Pat. No. 5,320,742. In the oxidative removal
of the mercaptans the mercaptans are converted to the corresponding
disulfides.
[0017] U.S. Pat. No. 5,510,568 discloses a process in which naphtha
is fed to a distillation column reactor which acts as a
depentanizer or dehexanizer with the lighter material containing
most of the olefins and mercaptans being boiled up into a
distillation reaction zone where the mercaptans are reacted with
diolefins to form sulfides which are removed in the bottoms along
with any higher boiling sulfur compounds.
SUMMARY OF THE INVENTION
[0018] Briefly, the present invention is a process for the
production of low benzene content gasoline comprising fractionating
a full boiling range naphtha to produce a light naphtha containing
olefins, a medium naphtha containing benzene and a heavy naphtha
containing toluene and xylenes and hydrogenating said medium
naphtha to convert the benzene to cyclohexane. The cyclohexane may
be isomerized to improve the octane. Preferably all of the
reactions are carried out under conditions of catalytic
distillation.
[0019] As used herein the term "catalytic distillation" means a
reaction carried out with a catalyst such that reaction and
distillation are going on concurrently. In a preferred embodiment
the catalyst is prepared as a distillation structure and serves as
both the catalyst and distillation structure.
BRIEF DESCRIPTION OF THE DRAWING
[0020] FIG. 1 is a flow diagram in schematic form of one embodiment
of the present invention.
[0021] FIG. 2 is a flow diagram in schematic form of a second
embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0022] The feed to the process comprises a benzene-containing
petroleum fraction which boils in the gasoline boiling range
(C.sub.5 to 450.degree. F. or full boiling range naphtha). The
fraction can be from a catalytic reforming unit or from a fluid
catalytic cracking unit. Generally the process is useful on the
naphtha boiling range material from catalytic cracker products
because they contain the desired olefins and heavier aromatic
compounds and the unwanted benzene. In addition the cracked
naphthas also contain unwanted sulfur compounds which are removed
in one embodiment of the invention. Straight run naphthas have very
little olefinic material, and unless the crude source is "sour",
very little sulfur.
[0023] Both full boiling range reformed naphtha and full boiling
range cracked naphtha have significant quantities of the heavier
aromatics.
[0024] The sulfur content of the catalytically cracked fractions
will depend upon the sulfur content of the feed to the cracker as
well as the boiling range of the selected fraction used as feed to
the process. Lighter fractions will have lower sulfur contents than
higher boiling fractions. The front end of the naphtha contains
most of the high octane olefins but relatively little of the
sulfur. The sulfur components in the front end are mainly
mercaptans and typical of those compounds are: methyl mercaptan
(b.p. 43.degree. F.), ethyl mercaptan (b.p. 99.degree. F.),
n-propyl mercaptan (b.p. 154.degree. F.), iso-propyl mercaptan
(b.p. 135-140.degree. F.), iso-butyl mercaptan (b.p. 190.degree.
F.), tert-butyl mercaptan (b.p. 147.degree. F.), n-butyl mercaptan
(b.p. 208.degree. F.), sec-butyl mercaptan (b.p. 203.degree. F.),
isoamyl mercaptan (b.p. 250.degree. F.), n-amyl mercaptan (b.p.
259.degree. F.), .alpha.-methylbutyl mercaptan (b.p. 234.degree.
F.), .alpha.-ethylpropyl mercaptan (b.p. 293.degree. F.), n-hexyl
mercaptan (b.p. 304.degree. F.), 2-mercapto hexane (b.p.
284.degree. F.), and 3-mercapto hexane (b.p. 135.degree. F.).
Typical sulfur compounds found in the heavier boiling fraction
include the heavier mercaptans, thiophenes sulfides and
disulfides.
[0025] The reaction of organic sulfur compounds in a refinery
stream with hydrogen over a catalyst to form H.sub.2S is typically
called hydrodesulfurization. Hydrotreating is a broader term which
includes saturation of olefins and aromatics and the reaction of
organic nitrogen compounds to form ammonia. However
hydrodesulfurization is included and is sometimes simply referred
to as hydrotreating.
[0026] The lower boiling portion of the naphtha which contains most
of the olefins is therefore not subjected to hydrodesulfurization
catalyst but to a less severe treatment wherein the mercaptans
contained therein are reacted with diolefins contained therein to
form sulfides (thioetherification) which are higher boiling and can
be removed with the heavier naphtha. The thioetherification reactor
can be either before or after a catalytic distillation
hydrodesulfurization reactor so long as the hydrodesulfurization
occurs in the stripping section of the catalytic distillation
hydrodesulfurization reactor such that the lower boiling point
materials are not contacted with the hydrodesulfurization
catalyst.
[0027] Thioetherification Catalysts
[0028] A suitable catalyst for the thioetherification reaction is
0.34 wt. % Pd on 7 to 14 mesh Al.sub.2O.sub.3 (alumina) spheres,
supplied by Sud-Chemie designated as G-68C. Typical physical and
chemical properties of the catalyst as provided by the manufacturer
are as follows:
2 TABLE I Designation G-68C Form Sphere Nominal size 7 .times. 14
mesh Pd. wt % 0.3 (0.27-0.33) Support High purity alumina
[0029] The catalyst is believed to be the hydride of palladium
which is produced during operation. The hydrogen rate to the
reactor must be sufficient to maintain the catalyst in the active
form because hydrogen is lost from the catalyst by hydrogenation,
but kept below that which would cause flooding of the column which
is understood to be the "effectuating amount of hydrogen" as that
term is used herein. Generally the mole ratio of hydrogen to
diolefins and acetylenes in the feed is at least 1.0 to 1.0 and
preferably 2.0 to 1.0.
[0030] The thioetherification catalyst also catalyzes the selective
hydrogenation of polyolefins contained within the light cracked
naphtha and to a lesser degree the isomerization of some of the
mono-olefins. Generally the relative rates of reaction for various
compounds are in the order of from faster to slower:
[0031] (1) reaction of diolefins with mercaptans
[0032] (2) hydrogenation of diolefins
[0033] (3) isomerization of the mono-olefins
[0034] (4) hydrogenation of the mono-olefins.
[0035] The reaction of interest is the reaction of the mercaptans
with diolefins. In the presence of the catalyst the mercaptans will
also react with mono-olefins. However, there is an excess of
diolefins to mercaptans in the light cracked naphtha feed and the
mercaptans preferentially react with them before reacting with the
mono-olefins. The equation of interest which describes the reaction
is: 1
[0036] This may be compared to the HDS reaction described below
which consumes hydrogen. The only hydrogen consumed in the removal
of the mercaptans in the present invention is that necessary to
keep the catalyst in the reduced "hydride" state. If there is
concurrent hydrogenation of the dienes, then hydrogen will be
consumed in that reaction.
[0037] HDS and Hydrogenation Catalyst
[0038] A preferable catalyst for the hydrogenation of benzene and
the destructive hydrogenation of the sulfur compounds
(hydrodesulfurization) is 58 wt % Ni on 8 to 14 mesh alumina
spheres, supplied by Calcicat, designated as E-475-SR. Typical
physical and chemical properties of the catalyst as provided by the
manufacturer are as follows:
3 TABLE II Designation E-475-SR Form Spheres Nominal size 8 .times.
14 Mesh Ni wt % 54 Support Alumina
[0039] Catalysts which are useful for either the hydrogenation of
benzene or the hydrodesulfurization reaction include Group VIII
metals such as cobalt, nickel, palladium, alone or in combination
with other metals such as molybdenum or tungsten on a suitable
support which may be alumina, silica-alumina, titania-zirconia or
the like. Normally the metals are provided as the oxides of the
metals supported on extrudates or spheres and as such are not
generally useful as distillation structures.
[0040] The catalysts may additionally contain components from Group
V and VIB metals of the Periodic Table or mixtures thereof. The use
of the distillation system reduces the deactivation and provides
for longer runs than the fixed bed hydrogenation units of the prior
art. The Group VIII metal provides increased overall average
activity. Catalysts containing a Group VIB metal such as molybdenum
and a Group VIII such as cobalt or nickel are preferred. Catalysts
suitable for the hydrodesulfurization reaction include
cobalt-molybdenum, nickel-molybdenum and nickel-tungsten. The
metals are generally present as oxides supported on a neutral base
such as alumina, silica-alumina or the like. The metals are reduced
to the sulfide either in use or prior to use by exposure to sulfur
compound containing streams.
[0041] The properties of a typical hydrodesulfurization catalyst
are shown in Table I below.
4 TABLE III Manufacturer Criterion Catalyst Co. Designation C-448
Form Tri-lobe Extrudate Nominal size 1.2 mm diameter Metal, Wt. %
Cobalt 2-5% Molybdenum 5-20% Support Alumina
[0042] The catalyst typically is in the form of extrudates having a
diameter of 1/8, {fraction (1/16)}or {fraction (1/32)} inches and
an L/D of 1.5 to 10. The catalyst also may be in the form of
spheres having the same diameters. In their regular form they form
too compact a mass and are preferably prepared in the form of a
catalytic distillation structure. The catalytic distillation
structure must be able to function as catalyst and as mass transfer
medium.
[0043] Isomerization Catalyst
[0044] Typically isomerization catalysts are of the Freidel Crafts
chlorided alumina catalyst having a Group VIII, particularly
platinum. Such catalysts are well known in the art and are
discussed in U.S. Pat. No. 4,783,575 which is incorporated herein
by reference.
[0045] Catalytic Distillation Structure
[0046] When the catalysts are used within a distillation column
reactor, they are preferably prepared in the form of a catalytic
distillation structure. The catalytic distillation structure must
be able to function as catalyst and as mass transfer medium. The
catalyst must be suitably supported and spaced within the column to
act as a catalytic distillation structure. Catalytic distillation
structures useful for this purpose are disclosed in U.S. Pat. Nos.
4,731,229, 5,073,236, 5,431,890 and 5,266,546 which are
incorporated by reference.
[0047] The most preferred structure is that shown in U.S. Pat. No.
5,730,843 which is incorporated by reference. As disclosed therein
the structure comprises a rigid frame made of two substantially
vertical duplicate grids spaced apart and held rigid by a plurality
of substantially horizontal rigid members and a plurality of
substantially horizontal wire mesh tubes mounted to the grids to
form a plurality of fluid pathways among the tubes. At least a
portion of the wire mesh tubes contain a particulate catalytic
material. The catalyst within the tubes provides a reaction zone
where catalytic reactions may occur and the wire mesh provides mass
transfer surfaces to effect a fractional distillation. The spacing
elements provide for a variation of the catalyst density and
loading and structural integrity.
[0048] Process Conditions
[0049] Conditions for the hydrogenation of benzene to cyclohexane
in a single pass downflow fixed bed reactor are known in the art.
Temperatures of about 400.degree. F. and pressures in the range of
300-500 psig are adequate when using a nickel catalyst. However,
copious quantities of hydrogen, especially between beds, are need
to control the temperature of the highly exothermic reaction.
Conditions in a distillation column reactor are considerably
different. Catalyst bed temperatures of between 250 and 300.degree.
F. at pressures of about 75 psig (about 30 psia hydrogen partial
pressure) are adequate. In addition the boiling of the liquid
within the bed dissipates the heat of reaction which is removed in
the overheads by condensation and reflux.
[0050] Process conditions for standard hydrodesulfurization of a
fluid cracked naphtha stream are also known. Temperatures in the
range of about 600-700.degree. F. along with pressures in the range
of 700-1000 psig and space velocities in the range of 1-10 volume
of naphtha per unit volume of catalyst. Hydrogen rates in the range
of 1000 to 1500 standard cubic foot per barrel of feed are commonly
used.
[0051] Process for the conditions for the hydrodesulfurization of a
heavy fluid cracked naphtha stream in a distillation column reactor
include temperatures in the range of 500.degree. F. and pressures
sufficient to keep a portion of the naphtha in the liquid state
(boiling) or about 200-300 psig. Hydrogen rates similar to that in
standard units are suitable.
[0052] Process conditions for the thioetherification of a light
naphtha in a standard fixed bed reactor are about 150 psig
pressure, about 300.degree. F., and a 10 WHSV (weight hourly space
velocity, in wt of feed per wt of catalyst per hour, hr.sup.-1).
About 6.25 volume of hydrogen per volume of feed is useful to keep
the catalyst in the hydride state.
[0053] Conditions in a distillation column used as a
thioetherification reactor include about 125 psig overhead
pressure, middle catalyst bed temperature of about 265.degree. F.
with hydrogen feed at about the same as for the standard
reactor.
[0054] The isomerization of cyclohexane to higher octane components
is disclosed in U.S. Pat. No. 4,783,575 which is incorporated
herein by reference. Generally the cyclohexane ring is first broken
and isomerized to isohexane. Any methyl cyclo pentane can also be
converted to isohexane. The conditions include 290-440.degree. F.
with pressures of about 370 psig. Space velocities of 0.5 to 3 are
preferred. Hydrogen is fed in an amount to provide a molar ratio of
from 0.01 to 10 moles of hydrogen per mole of hydrocarbon in the
outlet of the reactor. These conditions are suitable for both
standard fixed bed operation and distillation column reactor.
[0055] Referring now to the figures specific embodiments of the
process of the invention are shown.
[0056] In FIG. 1 there is shown a generic process scheme to produce
low benzene content gasoline while at the same time reducing the
sulfur levels. The full boiling range naphtha is fed via flow line
101 to a distillation column 10 having standard distillation
structure 12 contained therein. The standard distillation structure
may be sieve trays, valve trays, bubble cap or packing as is normal
in the industry. A light naphtha boiling below the boiling point of
benzene (about 175.degree. F. and lighter) is taken as overheads
via flow line 102. A mid range naphtha boiling in the range of
about 170 to 180.degree. F. is taken as a side draw via flow line
104. This mid range naphtha contains the benzene which has a
boiling point of 176.degree. F. The range is necessary to insure
that all of the benzene is removed. A heavy naphtha is boiling
above the boiling point of benzene (about 180.degree. F. and
heavier) is taken as bottoms via flow lined 103. The bottoms will
contain any octane rich toluene and xylenes.
[0057] The light naphtha may contain valuable olefins but also
diolefins and organic sulfur compounds which are mostly mercaptans.
To remove the mercaptans the naphtha can be subjected to a typical
sweetening process such as MEROX or by reaction with the diolefins
over a thioetherification catalyst 22 in a thioetherification
reactor 20. Hydrogen is added to the thioetherification reactor via
flow line 201 and product is removed via flow line 202.
[0058] The mid range naphtha in flow line 104 is subjected to
hydrogenation in a hydrogenation reactor 30 containing a
hydrogenation catalyst 32 with hydrogen being added via flow line
301. The mid range naphtha may also contain organic sulfur
compounds which would generally be thiophene in the boiling range
taken. The hydrogenation catalyst also acts as a
hydrodesulfurization catalyst to convert the thiophene to hydrogen
sulfide at the same time as the benzene is converted to
cyclohexane. The mid range naphtha having reduced benzene content
is removed via flow line 302 and may then be subjected to
isomerization catalyst 42 in isomerization reactor 40 to improve
the octane with product being removed via flow line 402.
[0059] The heavy naphtha in the bottoms may be subjected to
hydrodesuflurization in reactor 50 containing hydrodesulfurization
catalyst 52 with hydrogen being added via flow line 501.
Desulfurized heavy naphtha is removed via flow line 502. If desire
all of the product streams in flow lines 202, 402 and 502 can be
combined to produce a gasoline which is low in benzene and
sulfur.
[0060] Referring now to FIG. 2 a preferred embodiment of the
invention is shown. A first distillation column reactor 10 is shown
to contain a bed 12 of thioetherification catalyst in the
rectification section. The remainder of the column contains
standard distillation structure 13 as discussed above. The full
range naphtha is fed to the distillation column 10 via flow line
101 with the hydrogen necessary to keep the thioetherification
catalyst in the hydrided state being supplied via flow line 102.
The diolefins within the light naphtha react with the mercaptans to
form sulfides which are distilled downward and removed in the
bottoms. The now low sulfur light naphtha which boils below the
boiling point of benzene (about 170.degree. F. and lighter) is
removed as overheads via flow line 103.
[0061] A mid range naphtha boiling in the range of 170-180.degree.
F. is taken as a side draw via flow line 104 and fed to a second
distillation column reactor 20 which contains a bed 22 of
hydrogenation catalyst with hydrogen being fed flow line 201. The
rectification section of the distillation column contains standard
distillation structure 24 as discussed above. Any lighter naphtha
boiling below the boiling point of benzene or cyclohexane (about
174.degree. F. and lighter) and containing valuable olefins is
stripped out of the mid range naphtha and removed as overheads via
flow line 202 along with the hydrogen sulfide produced. The
remainder of the mid range naphtha containing the benzene and
possible thiophene is subjected to hydrogenation in the lower
portion of the column wherein benzene is converted to cyclohexane
and thiophene is converted to hydrogen sulfide. The mid range
naphtha now stripped of any lighter products and containing less
benzene and thiophene is removed a bottoms via flow line 203.
[0062] The bottoms from the hydrogenation distillation column
reactor in flow line 203 are fed to a third distillation column
reactor 30 containing a bed of 32 of isomerization catalyst and
standard distillation structure 34. In the reactor 30 the
cyclohexane is isomerized to higher octane product such as methyl
cyclopentane or isohexane. The advantage of the concurrent
distillation is that the isomerization product is removed from the
catalyst bed 32 as fast as it is formed thus improving the overall
production of the isomers. An overheads is taken via flow line 302
with bottoms containing the isomerization product being taken via
flow line 303. If hydrogen is needed, it is supplied via flow line
301. If necessary a second bed (not shown) of hydrodesulfurization
catalyst may be used to convert the thiophene to hydrogen
sulfide.
[0063] The bottoms, containing the heavy naphtha including the
heavier aromatic material such as toluene and xylenes is removed
via flow line 105 and fed to fourth distillation column reactor 40
containing a bed 42 of hydrodesulfurization catalyst. Hydrogen is
added via flow line 401. Standard distillation structure 44 may be
disposed above and below the bed 42. The heavier organic sulfur
compounds contained within the heavy naphtha are reacted with
hydrogen to produce hydrogen sulfide. The distillation is run not
so much for separation but to provide a condensing liquid within
the bed 42 which allows for use of lower hydrogen partial pressures
than otherwise be necessary. An overheads is taken via flow line
402 and a bottoms via flow line 403. The overheads liquid product
may be totally recycled as reflux after the hydrogen sulfide and
excess hydrogen are removed.
[0064] If desired all of the naphtha products in flow lines 202,
302, 303, 402 and 403 may be combined to produce a low benzene low
sulfur gasoline.
* * * * *